I know it sounds boring but all 3 are very good choices.
Treecko is grass type, making it very useful against Roxanne in the early gym (especcially if you speak to the guy at the end of Petalburg woods and get the Bullet Seed TM) and its evolutions great against Wallace in the later gyms.
Mudkip is my personal choice, its a water type at start making Roxanne a piece of cake. It evolves into a Water-Ground type making Grass its only true weakness, whats more, electric types don't effect him, making Wattson an easy win even for a low-level Marshtomp.
Torchic is fire, evolves into Fire-Fight. This makes it strong against Roxanne, once you know the fighting moves after evolution, and against Norman. Enough Said.
I'd also recommend catching a Zigzagoon, a Tailow a Poochyena and Wingull. That makes a perfect starter team and with Careful raising can be an effective team against the majority of the gyms.